560 SIMPLEST FORM OF GENERATIVE PROCESS. The termed the yolk-bag. The yolk-bag and its contents, namely the yolk and the germinal vesicle, constitute the ovum. eggs of many animals (as of Birds) contain an additional store of liquid albumen, the "white," enveloping the yolk-bag and destined to be gradually drawn into it, so as to replace the albumen of the yolk as it is progressively used-up in the development of the embryo; and the "shell" is a subsequent formation around this (§ 755). 734. The Hydra presents us with a very apposite illustration of the simplest mode in which the generative function is performed. Sperm-cells are developed at certain periods in the substance of its body near the origin of the arms; whilst ovules are evolved in the wall of the stomach nearer to the foot or base. By the rupture of the sperm-cells, their contained spermatozoids are set free in the surrounding water, and they penetrate to the ovules, which are exposed to their influence by the thinning-away of their exterior covering. From what has been observed in higher animals, there seems no reasonable doubt that the spermatozoids make their way through the germinal membrane, and penetrate into absolute contact with the germinal vesicle, which then lies near the surface of the ovule. What is the precise change effected by fertilization, has not yet been fully ascertained; the germinal vesicle, however, disappears; and it would seem as if its contents, with the product of the liquefaction of the spermatozoids, were diffused through the yolk, which soon begins to undergo changes of a very remarkable nature. 735. Before going further, it may be well to notice the remarkable antagonism which exists between the processes of Gemmation and Generation, as regards the conditions by which they are respectively favoured. For we see that in the Hydra, as in Plants, the extension of the body into buds is promoted by warmth and a copious supply of food; so that, as it would appear, if these be afforded, this mode of multiplication may be protracted indefinitely. On the other hand, if the supply of food be limited and the temperature lowered, the production of buds ceases, and the formation of sperm-cells and of germ-cells begins. The result is that ova or eggs are produced, which have a very firm horny covering, and possess a great power of resistance to cold; and thus provision is made for the continuance of the race through a winter tem EARLIEST STAGES OF DEVELOPMENT OF OVUM. 561 perature that might be fatal to the Hydra themselves. The same thing is observable among the Rotifera; for, as has long been known, two kinds of eggs are produced by them, the ordinary and the "winter eggs ;" and it now appears that the ordinary eggs, being evolved without any generative process, and with a rapidity proportional to the favouring influences of food and warmth, are really to be regarded as internal gemmæ ; whilst the "winter eggs," which are produced in the autumn by the concurrent action of males and females, and have a peculiarly dense horny investment, are the only true ova. Among the Aphides (§ 746), again, it has been experimentally shown that the non-sexual multiplication may be indefinitely protracted by warmth and food; whilst a reduction in the temperature and in the supply of nutriment causes this at any time to give place to sexual generation. 736. The first obvious change that presents itself in the Ovum, after its fertilization, is the "segmentation," or division of the yolk-mass into two halves, by the formation of a sort. of hour-glass contraction, which gradually deepens, until it produces a complete separation. Another segmentation of these two halves soon follows in the opposite direction, so that the yolk-mass becomes divided into four segments; each of these in its turn undergoes the like subdivision; and this. duplicating process is repeated, forming successively 8, 16, 32, 64, &c., segments, until a "mulberry-mass" is produced, which is composed of an aggregation of an immense number of minute yolk-spherules. Up to this stage, the developmental process takes place on essentially the same plan in all animals, save that in some the process of segmentation does not extend to the entire mass of the yolk, but only to a small proportion of it, which is distinguished as the "germ-yolk," whilst the remainder, which is applied to the nourishment of the more advanced embryo through an entirely different channel, is known as the "food-yolk" (§ 754). 737. It appears, among some of the simplest Worms, as if the "mulberry-mass" gradually shaped itself into the body of the animal, without the intervention of any intermediate structure; but in almost all Animals, the first stages in development tend to the production of a membranous expansion that may be likened to the "cotyledon" of Flowering Plants,— with this important difference, however, that whilst the latter 562 GERMINAL MEMBRANE.-DEVELOPMENT OF POLYPES. spreads itself out so as to come into contact with the "albumen of the seed by its external surface, the "germinal membrane" of the Animal forms itself around the yolk, and thus constitutes as it were a temporary stomach, within which the nutrient material is stored-up, and through the walls of which it is drawn into the embryo. This is accomplished in the following manner. The spherules of the outer layer of the mulberry-mass which are in immediate contact with the yolk-bag become invested with walls of their own, and thus become converted into proper cells; these are somewhat flattened and of a polygonal shape, very much resembling those of the epithelium of serous membrane (fig. 10). Another layer is afterwards formed within this, the cells of which retain more of their original globular form. But the spherules of the internal portion of the mulberry-mass, instead of becoming converted into cells, undergo dissolution and return to the condition of a liquid yolk; so that the ovum, in this stage of its development, consists of two layers of cells, constituting what are known as the "serous " and the "mucous layers of the germinal membrane, enclosing a mass of nutritious matter on which a change has been worked that seems to predispose it to become organized. 738. The development of the Polypes seems to advance but little beyond this point. The covering of the ovum bursts, and the contained embryo is set at liberty as soon as the germinal membrane has been formed around the yolk. In this state it becomes clothed with cilia, and is termed a gemmule; and it swims about freely in water for some time. Its form gradually becomes more elongated (fig. 303), tapering away at one end, which attaches itself after a time to some solid body; and its development into the polype-form soon commences. In the group of which the Hydra is an example, this change takes place in the following simple manner. The germinal membrane gradually thins away at the point furthest removed from the attached base, and at last an aperture is formed, which becomes the mouth; from around this aperture the tentacula or arms shoot forth, a single row being first formed, and others being afterwards added in those species in which they are numerous. Thus the two layers of the germinal membrane enter into the permanent structure of Fig. 303. DEVELOPMENT OF POLYPES AND MEDUSE. 563 the animal; the outer one constituting the external integument, and the inner becoming the lining of the stomach. The arborescent fabric of the composite Hydrozoa (§ 124) is gradually evolved by continuous gemmation from the original Polype; and whilst in some of them the sperm-cells and ova are developed within peculiar capsules not ostensibly differing (except in size) from the ordinary polype-cells, there are others in which they are the product of peculiar buds having the form and structure of Medusa, which buds in many instances become detached, and henceforth live as independent zooids, their sexual apparatus being only evolved after they have separated themselves from the parent stock. The sperm-cells and the ova are developed within different Medusa-buds; but both kinds of buds may (in many cases at least) be put forth from the same Polype-stock, as in monacious Flowering-Plants. 739. Although the two layers of the germinal membrane remain united in the Hydra and other Zoophytes formed upon its simple plan, they separate from each other at certain points in the Sea Anemone and its allies, so that a series of chambers is formed between them; and these chambers are afterwards set apart for the production of sperm-cells and germ-cells (§ 126). We do not meet in this group of Anthozoa with any example of that detachment of the sexual apparatus in the form of separate zooids, which is so remarkable a feature of the Hydrozoa. 740. The development of the Medusa, as elucidated by recent discoveries, presents several features of extraordinary interest. The sexes are distinct in these animals; spermcells being developed in some individuals, and ova in others, within the four chambers that surround the stomach (§ 120). When the ova have received the fertilizing influence, their first products are ciliated gemmules resembling those of Hydraform Polypes (fig. 304, a). These, after moving about for some time in the ovarial chambers of their parent, make their exit by the orifices of these, and then swim freely through the water. Gradually, however, they undergo the usual elongation, and fix themselves by one extremity (e); at the opposite extremity a depression appears in the middle, which is to become the mouth, as seen at b, and an elongation of the four corners (c, f) gives origin to the first tentacula, 564 DEVELOPMENT OF MEDUSÆ. which are afterwards increased by the addition of many others (g). In this manner a true polype is formed, which leads the life of a Hydra, and, like it, propagates its kind by the formation of polype-buds, which detach themselves and lead independent lives; and thus from a single Medusan egg there may arise a whole colony of polypes multiplied by gemmation. These differ entirely from Hydræ, however, in regard to their sexual apparatus, which is detached (as in the composite Hydrozoa) under an entirely different form, that of a Medusa. The body of រា Fig. 304.-DEVELOPMENT OF MEDUSA. the polype undergoes a great lengthening, and seems as if divided by transverse bands, which gradually deepen, so as to make the whole body almost resemble a pile of saucers with divided edges (h); for beneath the lowest of these saucer-like disks, a new set of polype - arms makes its appearance; and after the detachment of the whole pile of disks, the polype-body remains at their base, and may continue to lead its former life, and to propagate itself in the polype-form. The disks progressively enlarge, those at the summit of the pile increasing most rapidly, and then detaching themselves from the pile (i); when thus detached, they swim about freely in the water after the manner of the smaller and simpler Meduse, to which they closely correspond in form (d); and they gradually enlarge and acquire the structure of their original parents (k). It is not correct to represent (which is commonly done) the pile of Medusa-disks as being formed by the subdivision of the polype-body. The Medusa-disks are in reality sexual buds, resembling those of the composite Hydrozoa (§ 125); and the only essential difference between the two cases lies in the fact, that among the latter it is the Zoophytic form which ostensibly constitutes the animal (the Medusan buds being thrown-off only at certain times for a special purpose), whilst the former are only known (save to such as search-out the history of their polypoid development) in the Medusan stage of their lives. 741. The recent researches of Professor Müller and others |